UPDATED 10/29/10 5:57 p.m.

CHICAGO (CBS) - Two suspicious packages containing explosives were intercepted on their way from Yemen to Chicago, authorities said Friday. President Barack Obama said authorities had uncovered a “credible terrorist threat.”

Federal officials say two packages from Yemen that contained explosives were intercepted while still overseas. Both packages originated from Yemen and were addressed to two Chicago synagogues.

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One package was shipped through FedEx and was intercepted in Dubai. FedEx has told CBS 2 News that the suspicious package that tested positive for explosives never made it onto a FedEx plane. The package was detected by FedEx at its package handling facility in Dubai.

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The second package was discovered aboard a UPS cargo plane in East Midlands, north of London, and contained a printer toner cartridge with wires and powder (pictured above). It was found during routine screening of cargo in England, prompting authorities to scour three planes and a truck in the United States on Friday, U.S. officials said.

“They were not letters, they were larger than that; think bread boxes or so, in terms of size,” said White House counter-terrorism adviser John Brennan. “We don’t know yet how they were intended to be activated. There are a number of things we know and we’re piecing together those puzzle pieces and working very closely with the British and Emirate authorities. At this point, there is a lot of forensic work that needs to be done.”

The white powder, according to law enforcement sources, was the same substance used by the Christmas Day bomber flying into Detroit. Its called TATP; it is explosive but needs an ignition source to become a bomb.

President Barack Obama said Friday afternoon that authorities had uncovered a “credible terrorist threat” against the United States and that initial examination of the two packages determined that they contained explosive materials.

Obama said the packages originated in Yemen and were apparently bound for places of Jewish worship in Chicago.

The events “underscore the necessity of remaining vigilant against terrorism,” the president said. The packages both originated in Yemen, but Obama did not assign blame to al-Qaida, which is active in the Arab nation and long has made clear its goal of attacking the United States.

Indications were that the goal was psychological, rather than physical damage.

“Certainly was a simple attempt to disrupt rather than a dry run. And i think it was an indication of the tremendous expenses which we are bearing in order to prevent ourselves from being struck by terrorists in order to protect the security of the American people,” University of Chicago professor Marvin Zonis said. “Certainly, it’s psychological terrorism. The goal is to disrupt us, to frighten us, but it isn’t the endgame. They’d rather get an explosive into our country and blow up some people, kill some people.”

The discoveries led to emergency cargo plane checks in Philadelphia and Newark Friday morning.

UPS cargo planes which reportedly contained packages from Yemen were taken to isolated areas of both Newark’s Liberty International Airport, and Philadelphia’s international Airport, where they were thoroughly searched before being allowed to continue their trips.

Another suspicious package was discovered on a truck crossing one of the bridges into New York City.

In Chicago, cargo terminals for both UPS and FedEx at O’Hare International Airport were immediately swept with bomb-sniffing dogs, and security around them was tightened as well.

There was no evidence found that any explosives reached Chicago or that they could have caused any damage if they did.

The events unfolded four days before national elections in which discussion of terrorism has played almost no role.

Obama said the U.S. is taking steps to enhance the safety of air travel, including beefing up cargo screening.

Obama told reporters Friday that he was alerted to the threat Thursday night and that he directed the Homeland Security Department and other law enforcement and intelligence agencies to take whatever steps were necessary to protect American citizens.

He said additional protective measures will be taken for as long as necessary to ensure safety and security in this country.

Officials suspect a link with al-Qaida.

The Jewish Federation of Metropolitan Chicago was notified of the situation at 10:30 a.m. Friday, and officials there are “taking proper precautions,” said associate vice president Linda Haase.

The organization is also advising local synagogues to take precautions, Haase said.

A law enforcement source tells CBS 2 Chief Correspondent Jay Levine the word is out to synagogues in Chicago not to accept UPS packages from east of New York. The are asked to call Chicago Police if any delivery is attempted.

FBI spokesman Ross Rice added that while there are no identifiable or specific threats to the Chicago area, all churches, synagogues and mosques in the area are being warned to be vigilant for unsolicited or unexpected packages, especially those originating from overseas locations.

Obama was notified of the potential terrorist threat Thursday evening, and “directed U.S. intelligence and law enforcement agencies, and the Department of Homeland Security, to take steps to ensure the safety and security of the American people, and to determine whether these threats are a part of any additional terrorist plotting,” according to a statement from White House press secretary Robert Gibbs. “The president has received regular updates from his national security team since he was alerted to the threat.”

The president was going ahead with plans to head home to Chicago on Saturday for a political rally for local Democratic candidates. The synagogue near his Hyde Park home isn’t one of those targeted in the plot, so authorities saw no need to cancel his travel plans.

“Whenever the president travels we take a very careful look at what the threat environment might be,” Brennan said. “At this point there is no effect (to Obama’s plans).”

CBS 2 Chief Correspondent Jay Levine, The Associated Press and the Sun-Times Media Wire contributed to this report.

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